For over three decades, the electron microscope has been an integral part of the research program of the Epidemiology Section regarding the etiologic role, natural history and physical characteristics of various important enteric viruses. Past highlights of these investigations have included inquiry into the etiology of diseases of unknown cause involving studies of fastidious agents of disease that defied cultivation in any cell culture system. The electron microscope played a major role (i) in the discovery of the 27nm Norwalk virus (now known as a member of the norovirus genus in the calicivirus family) and the establishment of its etiologic role in human diarrheal disease, and (ii) in collaboration with the Hepatitis Viruses Section, in the discovery of the 27nm hepatitis A virus particle and establishment of its etiologic role in infectious hepatitis. It also was used as an important epidemiologic tool for the detection of rotaviruses in stools of infants and young children hospitalized with diarrhea and helped in establishing the importance of these agents in severe diarrhea of infants and young children. Although intensive efforts have been made, the Norwalk and related human enteric noroviruses have yet to be cultivated in any tissue culture system. This, along with the expression of 27nm virus-like particles of certain members of this group of agents have contributed to the continuing important role of the electron microscope in the research program of the Epidemiology Section. The electron microscope has continued to be an important primary and adjunctive tool in various capacities. For example, since the previous annual report it continued to be used for various studies including examination of: (1)virus-like particle (VLP) preparations prepared from Children's Hospital archival GII-4 specimens;(2) a high-titered tetravalent human-bovine rotavirus (UK) reassortant vaccine preparation from a licensee to determine if any contaminant could be seen, to assist in an investigation of the cause of guinea pig toxicology issues;(3) VLP preparations of various caliciviruses (Norwalk virus, MD 145 virus, mink calicivirus, murine norovirus and others);(4) stool specimen from a chimpanzee with non A-E hepatitis by IEM with a pre and two post infection sera in an attempt to detect a viral agent (in collaboration with Dr. Purcell);(5) a virus seed of a primary African Green monkey kidney grown human-bovine reassortant rotavirus in an investigation of an adventitious agent;(6) purified dengue 4 vaccine preparations (in collaboration with Dr. Anez Gutierrez);(7) hepatitis E virus preparation (in collaboration with Dr. Emerson).